Physicochemical tuning and possible Moss–Burstein band modulation in Eco-sustainable PVA/ZnO-CoFe₂O₄ magnetoactive hybrid nanocomposite films for remote actuation

Eco-sustainable magnetoactive hybrid nanocomposite films based on polyvinyl alcohol (PVA) and reinforced with ZnO and CoFe₂O₄ nanoparticles were fabricated via a green-assisted solution-casting approach to investigate coupled structure–property–function relationships. Structural analysis revealed a composition-dependent evolution of crystallinity: ZnO-rich composition (HC1) exhibited increased amorphous disorder, intermediate composition (HC2) showed partial recovery of crystallinity, and CoFe₂O₄-rich system (HC3) induced strong polymer chain confinement. X-ray diffraction confirmed the formation of wurtzite ZnO and spinel CoFe₂O₄ phases, with slight peak shifts indicating interfacial interaction and lattice modification. Optical analysis demonstrated significant band-gap modulation, with the secondary (lower-energy) apparent optical band gap decreasing from 3.43 eV for pure PVA to 2.35 eV for HC1 and subsequently increasing to 2.67 eV for HC3, while the primary (higher-energy) transition decreased from 5.21 eV for pure PVA to 3.42 eV for HC1 and then progressively increased to 3.83 eV for HC3, indicating defect-state formation accompanied by an apparent composition-dependent widening of the optical band gap. The correlated optical and electrical behavior is consistent with a possible Moss–Burstein-type band-filling contribution, although additional mechanisms such as defect-state evolution and interfacial charge transfer may also contribute. These variations are attributed to the combined influence of nanofiller phase fraction, structural disorder, defect-state density, nanoparticle dispersion, interfacial interactions, and polymer-chain confinement. Dielectric and AC conductivity measurements revealed thermally activated hopping conduction with distinct temperature regimes. The extracted E a values decreased with frequency from 1.746–0.729 eV to 0.208–0.086 eV for pure PVA, 0.763–0.499 to 0.262–0.217 eV for HC1, 0.563–0.336 to 0.393–0.325 eV for HC2, and 0.649–0.471 to 0.355–0.222 eV for HC3 across their respective low- and high-temperature regions, indicating enhanced short-range hopping conduction. A systematic reduction in activation energy with increasing frequency (1 kHz–1 MHz) confirmed a transition from long-range to localized transport. Magnetic measurements demonstrated enhanced ferromagnetic behavior and exchange bias, while magnetic actuation experiments revealed efficient field-induced bending of HC3 films under applied magnetic fields up to 800 G, demonstrating a pronounced magneto-mechanical coupling. These results demonstrate that nanoparticle dispersion, interfacial polarization, and band-structure modulation collectively govern the hybrid system's multifunctional behavior. The study establishes a comprehensive framework for designing advanced polymer nanocomposites with tunable optoelectronic and magnetoactive properties for applications in flexible electronics, sensing, and smart actuators.